Field of the Invention
The present disclosure relates to an optical member having good optical properties and a method for manufacturing the same.
Description of the Related Art
It has been known that optical elements are provided with an antireflection film including an optical film having a varying refractive index or a plurality of optical films having different refractive indices to prevent reflection at the interface of the optical element through which light enters or is emitted. Such an antireflection film is formed by vacuum deposition, such as vapor deposition or sputtering, or wet film forming, such as dip coating or spin coating.
In general, the surface layer of the antireflection film is formed of a transparent material having a low refractive index. such materials include inorganic materials such as silicon oxide, magnesium fluoride, and calcium fluoride, and organic materials such as silicone resin and amorphous fluororesin.
It has recently been known to use a low refractive index film containing air, whose refractive index is 1.0, as the antireflection film. By forming pores in a silicon oxide or magnesium fluoride layer, the refractive index of the film can be reduced. For example, by forming pores with a percentage of 30% by volume in a magnesium fluoride thin film having a refractive index of 1.38, the refractive index can be reduced to 1.27.
For increasing the number of pores in a known process, silicon oxide hollow particles or nonspherical silicon oxide particles such as chainlike silicon oxide particles are used.
Japanese Patent Laid-Open No. 2010-217699 discloses an antifouling low refractive index film made up of silicon oxide particles and a binder containing fluorine compound.
U.S. Pat. No. 8,088,475 discloses an anti-reflecting membrane including a layer made of particles, coated with a liquid-repellent surface layer made of a fluorine compound.
Unfortunately, the low refractive index film disclosed in Japanese Patent Laid-Open No. 2010-217699 contains the fluorine compound with a high proportion to the silicon oxide particles to impart an antifouling property and, accordingly, exhibits a high reflectance.
In The antireflection film disclosed in U.S. Pat. No. 8,088,475, the particle layer is not completely covered with the fluorine compound. If dirt is attached to the particle layer, the dirt will spread through the pores among the particles and thus degrade the optical properties of the membrane.